1. Field of the Invention
This invention relates to a process of manufacturing a catalyst for purifying the exhaust gases from internal combustion engines, which catalyst comprises a skeleton comprising sintered alumina and at least one stabilizing rare earth element oxide and is preferably provided as a coating on a metallic, mineral or ceramic support, and which catalyst comprises at least one noble metal, which is finely dispersed on the skeleton.
2. Description of the Prior Art
Such process has been disclosed in Published German Application No. 3,223,500. In that known process, active alumina is impregnated with an aqueous solution of a cerium salt and is then dried and heated to 300.degree. to 700.degree. C. in contact with air for 20 to 180 minutes in order to convert the cerium salt to cerium oxide. The aluminum oxide substrate doped with cerium oxide is subsequently impregnated with an aqueous solution of a noble metal salt and is then dried and subsequently calcined at 250.degree. to 640.degree. C. in contact with air in order to expel the solvent, decompose the salt and bond the remaining noble metal to the doped alumina substrate. The substrate may be provided in the form of shaped elements of any desired geometric configuration, such as spheres, cylinders, rods, rings etc.
In many cases the commercial catalyst comprises the alumina on a honeycomb support, which consists of a metal, such as iron, steel or zirconium, or of a ceramic oxide, such as thorium oxide, or of a mineral ceramic material, such as cordierite.
It has been disclosed in Published German Application No. 3,223,500 that such catalysts provided with a separate support can be manufactured in a process in which an aqueous of dispersion of active alumina is prepared first, which dispersion also contains dissolved cerium salt or dispersed cerium oxide, said dispersion is applied in consecutive layers on the honeycomb support and is dried thereon and is subsequently calcined or baked at 300.degree. to 700.degree. C., and the resulting support coated with alumina is finally impregnated with a solution of a noble metal salt, subsequently dried, and calcined at 250.degree. to 650.degree. C. in contact with air.
In order to promote a preferential deposition of the noble metal on the surface of the support, it has been proposed in Published German Application No. 3,223,500 to impregnate the supports with demineralized water in an amount corresponding to 45% of their pore volume before they are impregnated with the solution of the noble metal salt.
Such catalysts are used to purify the exhaust gases of internal combustion engines by an oxidation of the carbon monoxide and hydrocarbons and by a reduction of nitrogen oxides contained in the exhaust gas. For this purpose the catalysts must be suitable for operation at high temperatures. The catalysts described in Published German Application No. 3,223,500 can be operated at temperatures up to about 950.degree. C. Higher temperatures will result in an undesired growth of the particles of alumina and noble metal because they are partly sintered further to form larger particles. That result is highly undesirable because it involves a decrease of the effective surface area of the catalyst and of the conversion capacity of the catalyst. Without the cerium oxide, which in accordance with Published German Application No. 3,223,500 is added to stabilize the alumina skeleton by opposing the recrystallization and sintering of the alumina, such catalyst cannot be operated at temperatures in excess of about 900.degree. C.